The present invention relates to an image forming apparatus, such as a color electronic copying machine, which reads an image on an original and forms an image using an electrophotographic process.
In recent years, color images have frequently been handled in offices. An increasing number of electrophotographic image forming apparatuses, in particular, color printers and color multi-functional peripherals (color MFPs), have been installed in offices.
At present, in many offices with a certain scale, a black-and-white copying machine and a color printer are used in combination. As color printers have increasingly gained popularity among smaller-scale offices, there is a demand for color MFPs, which could replace black-and-white copying machines.
In this case, it is desired that the color MFP have the same black-and-white copying performance as the black-and-white copying machine and also have a color print function. At present, a most prevalent system employed in color image forming apparatuses is one using an intermediate transfer medium. Images of four colors are successively developed on a photosensitive body, and the developed images are successively transferred on the intermediate transfer medium. The superimposed images of the four colors on the intermediate transfer medium are batch-transferred on a paper sheet.
In this type of apparatus, in most cases, a secondary transfer member that comes in contact with the back side of the paper sheet is used in order to batch-transfer the images on the sheet. In case of paper jam, the secondary transfer member comes in direct contact with the intermediate transfer medium carrying the toner images, and the secondary transfer member is stained with toner. Consequently, the back side of a paper sheet for the next print operation is also stained. In the case of double-side printing, both the image surface and back surface of the paper sheet are stained.
Some methods for removing the stain have been proposed. Such methods include a method wherein toner of a stain is mechanically removed by a blade, etc., and a method wherein an electric field is applied to toner of a stain and the toner is recovered to the intermediate transfer medium side.
In particular, the method using the electric field has such a problem that the toner cannot efficiently be recovered by simple application of an electric field.
The object of an aspect of the present invention is to provide an image forming apparatus wherein an electric field is applied to toner stain on a secondary transfer member, thereby efficiently recovering the toner to an intermediate transfer medium side.
In order to achieve the object, the present invention may provide an image forming apparatus having image forming means for forming a plurality of toner images on a movable image carrying body, an intermediate transfer member on which the toner images formed by the image forming means are transferred, and a transfer rotary body that transfers the toner images, which are transferred on the intermediate transfer member, onto a transfer medium, the apparatus comprising: a first application unit that applies a negative voltage to the transfer rotary body; a second application unit that applies a positive voltage to the transfer rotary body; a switching unit that alternately switches the first application unit and the second application unit, which apply voltages to the transfer rotary body; and a control unit that effects a control to set a time period of alternate voltage application switched by the switching unit to be less than a time period corresponding to a single continuous rotation of the transfer rotary body relative to the intermediate transfer member.
Additional objects and advantages of an aspect of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out hereinafter.